The present invention relates to a method for detecting knocking combustion in the operation of an internal combustion engine.
In the case of externally ignited internal combustion engines, the air/fuel mixture is already ignited during the compression stroke, before the upper dead center, because of the finite propagation speed of the combustion. The optimal ignition point is thereby determined as a function of the crankshaft speed. Knocking combustion occurs as a result of premature ignition, in which excessive pressure peaks lead to an increase in the temperature of the unburned air/fuel mixture and thus to local self-ignitions of the air/fuel mixture. These excessively high pressure peaks in the combustion chambers occur as a rule during full load operation of the internal combustion engine.
In order to determine the knocking combustion, the course of the combustion chamber pressure is recorded by measuring the ionic current in all combustion chambers of the internal combustion engine. A method for measuring the ionic current is known, for example, from DE 33 27 766 A1. In which knocking combustion is detectable by the occurrence of pressure peaks and oscillations in the course of the combustion chamber pressure. To do this, an alternating voltage is fed to the spark plug, which simultaneously acts as an ionic current probe, immediately after the ignition of the air/fuel mixture via the ignition coil. This alternating voltage is modulated by ions in the burning air/fuel mixture as a function of the combustion chamber pressure. The modulated alternating voltage, the raw ionic current signal, is fed to an evaluation circuit.
The ionic current cannot be measured during the ignition of the air/fuel mixture by the spark plug. A method is described in DE 197 20 535 A1 in which the spark duration of the spark plug is reduced to a minimum in order to record the course of the combustion chamber pressure directly afterwards and to detect any knocking combustion.
DE 197 20 532 C2 discloses that, for the evaluation, the raw ionic current signal first has to be filtered through a bandpass filter. Then the part of the filtered signal not caused by knocking is masked by windowing. The remaining signal portions are rectified and then integrated. The resulting knock integral value is proportional to the intensity of the pressure peaks in the course of the combustion chamber pressure.
If knocking combustion is detected in an internal combustion engine, the ignition point for the coming work stroke is shifted within the compression stroke nearer to the upper dead center. Nevertheless, internal combustion engines develop their maximum power with light knocking, whereas with heavier knocking there is a danger of damage occurring to the internal combustion engine with the passage of time. It is thus necessary to specify a threshold value for the knocking integral value, up to which the internal combustion engine can be operated with light knocking, and from which the ignition point must be shifted towards the upper dead center.
The problem with the evaluation of an ionic current signal lies in the large fluctuations in the intensity which occur during the measurement of the ionic current signal from work stroke to work stroke.
Such fluctuations in intensity can have the effect that measured raw ionic current signals with low intensity, which have large pressure peaks and oscillations caused by knocking, give such low knock integral values that they remain below the threshold value for the knock integral, and the ignition point is thus not shifted towards the upper dead center.
Similarly, measured raw ionic current signals with high intensity, which have low pressure peaks and oscillations caused by knocking, give such high knock integral values that they lie above the threshold value for the knock integral, and the ignition point is thus shifted towards the upper dead center, as a result of which the internal combustion engine does not develop its maximum power.
In the basic background publication DE 42 32 845 C2, it is recommended to form an average value in order to improve the assessment of the knock integral value, in which knock integral values are summed and the sum is then divided by the number of the integral values.
The disadvantage of this procedure is that whereas the knock integral values can be better assessed, the fluctuations in the intensity of the raw ionic current signal are still contained in the average value of the knock integral values.
The object of the invention is to specify a method for detecting knocking combustion during the operation of an internal combustion engine which guarantees a reliable assessment of the knock integral value and which compensates for fluctuations in the intensity of the raw ionic current signal.
The above object is achieved according to the invention in a method for detecting knocking combustion during the operation of an internal combustion engine, by obtaining a raw ionic current signal with an ionic current probe during a combustion stroke in a combustion chamber of the engine, and then evaluating the raw ionic current signal to determine a knocking intensity. Particularly according to the invention, fluctuations in the intensity of the raw ionic current signal are compensated for in the assessment of a knock integral value by normalizing the raw ionic current signal or the value of the knocking intensity
The chronological course of the raw ionic current signal is composed first of the ignition faults, then of a first maximum of the combustion, and then of a second maximum of the ionic current which is proportional to the pressure and temperature of the combustion chamber.
To perform the normalization, an ionic current amplitude value, that is the maximum value of the second maximum of the raw ionic current signal, or an ionic current integral value, that is the value of the integrated raw ionic current signal, is determined currently from the raw ionic current signal after each combustion stroke.
Furthermore, to perform the normalization while taking average values into consideration, the quotient is formed from averaged ionic current amplitude values of the previous combustion strokes and a current ionic current amplitude value, or the quotient is formed from averaged ionic current integral values of the previous combustion strokes and a current ionic current integral value.
As normalization for the assessment of the knock integral value, the raw ionic current signal or the value of the knocking intensity is divided by the ionic current amplitude value.
Alternatively, as normalization for the assessment of the knock integral value, the raw ionic current signal or the value of the knocking intensity can be divided by the ionic current integral value.
Two additional normalizations for the assessment of the knock integral value consist of forming the quotient from the averaged ionic current amplitude value and the ionic current amplitude value multiplied by the raw ionic current signal or multiplied by the value of the knocking intensity.
Two additional alternative normalizations for the assessment of the knock integral value consist of forming the quotient from the averaged ionic current integral value and the ionic current integral value multiplied by the raw ionic current signal or multiplied by the value of the knocking intensity.
The raw ionic current signal is evaluated, at least in the process steps:
Filtering, as a result of which only the oscillations of the knocking or the ignition remain as a signal from the raw ionic current signal.
Windowing, as a result of which only the oscillations of the knocking remain as a signal.
Rectification, as a result of which negative signal values are avoided. Integration, as a result of which the knocking intensity value is formed.
In a development of the invention, it is intended to use the spark plugs of the internal combustion engine as ionic current probes.
In a further development of the invention, it is intended that the averaging of the ionic current amplitude value or of the ionic current integral value is performed as a sliding average.
To form the sliding average, the average of the ionic current amplitude or of the ionic current integral of the previous combustion strokes is subtracted from the current ionic current amplitude value or from the current ionic current integral value, this difference is multiplied by a weighting factor, and this product is then added to the average value of the ionic current amplitude or to the ionic current integral of the previous combustion strokes.
A reliable assessment of the knock integral value is provided by the method according to the invention for detecting knocking combustion during the operation of an internal combustion engine, as a result of which it is possible to operate an internal combustion engine at its maximum power.
The method for detecting knocking combustion during the operation of an internal combustion engine, in which fluctuations in the intensity of the raw ionic current signal are compensated by normalization of the raw ionic current signal or the value of the knocking intensity, is shown and explained in the following by means of embodiments and illustrated by two figures.